Phenoxy resins of various molecular weights are commercially available for use in a variety of applications such as adhesives, coatings, moldings, and blow-molded bottles. Excellent reviews on phenoxy resins are available. For example, W. F. Hale, "Phenoxy Resins", Encyclopedia of Polymer Science and Technology, Vol. 10, p. 111 (1969) (edited by H. F. Mark, N. G. Gaylord and N. M. Bikales) and K. Neville, Ch. 2, "Phenoxy Resins", New Linear Polymers, p. 17, Mc-Graw Hill Book Co., N.Y., 1967. Linear phenoxy resins are extremely sensitive to certain solvents and have a maximum operating use temperature under load of approximately 80.degree. C. The solvent resistance and the use temperature of phenoxy resins have been improved as indicated in the literature by reacting pendent hydroxy groups with various crosslinking reactants such as anhydrides, epoxies and diisocyanates. The method for reacting phenoxy resins with various crosslinking agents varies. Generally the phenoxy resin is dissolved in a solvent and the crosslinking agent added to the solution. The shelf-life (stability) of the solution will vary depending upon the concentration and the nature of the crosslinking reagent. Limited shelf-life is a major problem using these crosslinking reagents and generally necessitates two-part systems. Other disadvantages of these crosslinking methods include significantly more difficult processing especially in injection and compression molding, higher moisture absorption due to hydrophilic polar sites, lower toughness, and lower thermal stability, particularly from diisocyanate crosslinking. There is thus a definite need in the art for phenoxy resins having pendent reactive crosslinking groups and improved shelf-life properties.
Accordingly, it is an object of the present invention to provide a new class of phenoxy resins having pendent ethynyl groups.
Another object of the present invention is to provide a new class of phenoxy resins that have improved shelf-life properties.
Another object of the present invention is a phenoxy resins class that crosslinks upon the application of heat to yield a cured resin having improved solvent resistance and higher use temperature that linear uncrosslinked phenoxy resins.
A further object of the present invention is a novel phenoxy resin having improved processability, toughness, solvent resistance and increased use temperature and the process for preparing same.
According to the present invention the foregoing and additional objects are attained by reacting phenoxy resins with ethynyl-substituted aryl acid chlorides such as 4-ethynylbenzoyl chloride to provide pendent ethynyl crosslinking groups to the phenoxy resin. The ethynyl content can be readily controlled simply by altering the amount of 4-ethynylbenzoyl chloride used in the reaction or by co-reaction of the pendent hydroxy groups on the phenoxy resins with mixtures of the ethynyl-substituted aryl acid chloride and other acid chlorides, e.g., benzoyl chloride. In controlling the ethynyl content, the crosslink density of the cured resin can be cured accordingly. Also, if desired, residual hydroxy groups can be left in the phenoxy resin without encountering any major disadvantages. High ethynyl content provides high crosslink density in the cured resins. Also, the properties of solvent resistance, processability and toughness are controlled by the ethynyl content and accordingly, the crosslink density. Phenoxy resins containing high ethynyl content are more difficult to process than those with lower ethynyl content. Conversely, the higher the ethynyl content, the better the solvent resistance of the cured resin. The synthesis of phenoxy resins containing pendent ethynyl groups can be readily controlled to adjust the properties of the cured resins for specific applications.
Phenoxy resins are relatively high molecular weight amorphous engineering thermoplastics synthesized from the reaction of diphenols and epichlorohydrin using a strong base. An exemplary phenoxy resin used in the specific examples described is PKHH, a commercial product available from the Union Carbide Corporation and has the following chemical structure: ##STR1## where n=20-200 repeating units.
Linear phenoxy resins are reported in the literature as having good toughness, relatively high modulus, moderate thermal stability, excellent moldability, high abrasion resistance and good adhesive strength. The major uses for linear phenoxy resins are as molding materials, coatings and adhesives. The chemical resistance of a phenoxy resin is dictated by the chemical structure and the amorphous nature of the polymer. These resins are resistant to acids and bases but soluble or become swollen in polar solvents such as ketones, esters, and chlorinated hydrocarbons. To alleviate this problem and gain better weathering characteristics, several approaches have been utilized to crosslink these resins, such as the pendent hydroxyl groups having been crosslinked by anhydrides, diisocyanates, urea- and melamine-formaldehyde resins. These methods usually employ two component systems and produce polymers that are unstable in solution. The present invention, however, modifies a phenoxy resin by incorporating a thermally reactive ethynyl group pendent to the polymer chain resulting in a polymer that can be crosslinked when heated to produce a material having improved solvent resistance and higher softening temperature. By controlling the quantity of ethynyl groups the processability of phenoxy resins is affected as well as the cured resin solvent resistance and glass transition temperature or use temperature. The 4-ethynylbenzoyl chloride employed in the present invention was obtained by reacting methyl-4-bromobenzoate with trimethylsilylacetylene using a palladium catalyst and the trimethylsilyl group was subsequently cleaved with potassium carbonate. After saponification, the carboxylic acid was converted to the acid chloride as previously reported in the Journal of Polymer Science, Polymer Chemistry Edition, 20, 3131 (1982). 4-ethynylbenzoyl chloride was obtained as a yellow crystalline solid, m.p. 75.degree.-76.degree. C. after recrystallization from hexane.
A more complete description of the preparation of the 4-ethynylbenzoyl chloride is found in Hergenrother's U.S. Pat. No. 4,431,761, issued Feb. 14, 1984.
Generally, to attach the pendent ethynyl groups onto the phenoxy resins according to the present invention, 4-ethynylbenzoyl chloride or a mixture of benzoyl chloride and 4-ethynylbenzoyl chloride was added to a solution of the phenoxy resin, e.g., PKHH, (Union Carbide Corporation) containing a small amount of triethylamine. After stirring overnight, the polymer was precipitated in methanol, boiled in fresh methanol, and dried in vacuo at 80.degree. C. to recover an off-white solid. The recovered polymers were dissolved in CHCl.sub.3 and stirred to assure thorough mixing. The resulting blends were then air dried at ambient temperature for eighteen hours and subsequently dried at 70.degree. C. for four hours. To obtain films polymer solutions were made in CHCl.sub.3 (20% weight/volume) and doctored onto plate glass, stage dried in a circulating air over to a final temperature of 250.degree. C. and held for one-half hour at 250.degree. C.
Having generally described the invention, a more complete understanding thereof can be obtained by reference to the following specific examples which are provided herein for purposes of illustration only and are not to be limiting on the invention.